Recent years have witnessed a strong demand for lighter and thinner batteries as the power supplies of, for example, portable devices. The covering material of batteries has also come to employ materials such as metal foil and laminate films in which a metal foil and heat-seal resin film are laminated, and these films enable both lighter and thinner structures and greater freedom in the shapes that are adopted.
A three-layer laminate film can be taken as a representative example of a laminate film that is used as the outer covering of a battery, this laminate film being formed by laminating heat-seal resin film, which is a heat-seal layer, on one side of an aluminum thin-film, which is the metal foil, and then laminating a protective film on the other surface.
As shown in FIG. 1, in a film-covered battery that uses a laminate film as the covering material, battery element 106, which is composed of positive electrodes, negative electrodes, and an electrolyte, is typically interposed between two laminate films 103 and 104 in which the heat-seal resin films confront each other, following which laminate films 103 and 104 are heat-sealed around the periphery of battery element 106 (the area indicated by diagonal lines in the figure) to hermetically seal (hereinafter referred to as simply “seal”) battery element 106.
Protruding tabs are provided on each of the positive electrodes and negative electrodes to lead out from the positive electrodes and negative electrodes of battery element 106, and collectors 107a and 107b, in which the tabs are collected for each electrode, are in turn connected to lead terminals 105a and 105b, respectively, that extend out from laminate films 103 and 104. In addition, at least one of laminate films 103 and 104 is formed as a flanged receptacle by deep-drawing so as to facilitate the accommodation of battery element 106.
The heat-sealing of the laminate films is carried out by applying pressure and heat to laminate films 103 and 104 by means of a pair of heat-seal heads 109a and 109b, as shown in FIG. 2.
At this time, the heat that is applied by heat-seal heads 109a and 109b is also conveyed to the area surrounding the points of laminate films 103 and 104 that are to be heat-sealed, with the result that heat-seal resin 103d and 104d is melted in areas in which heat-sealing is not required. When heat-seal resin 103d and 104d is melted in portions A and B that contact battery element 106, battery element 106 contacts metal foil 103e and 104e of laminate films 103 and 104, raising the concern for short-circuits between these two surfaces.
In JP-A-2001-126678, a battery is disclosed that is directed to the prevention of short-circuiting by arranging a heat-seal resin film of the same material as the heat-seal resin at the points of the laminate films that are heat-sealed and the vicinities of these points to substantially increase the thickness of the layers of heat-seal resin at points susceptible to short-circuiting.
On the other hand, JP-A-2001-6633 discloses a technology for improving the heat resistance of the laminate films according to which the battery element is sealed in a laminate film, following which the areas of the laminate films that have undergone heat-sealing are subjected to irradiation by an electron beam to form cross-linked structures in the heat-seal resin and thus improve the reliability of sealing.
However, the technique that is disclosed in JP-A-2001-126678 involves nothing more than increasing the thickness of portions of the heat-seal resin layer, and brings about no particular change regarding the melting of heat-seal resin in areas susceptible to short-circuiting that are in the vicinities of points that are heat-sealed when sealing the battery element. Accordingly, if heat-seal conditions are not appropriately set according to the thickness of the layer of the heat-seal resin, either adequate heat-sealing may not be achieved, or conversely, excessive melting of the heat-seal resin may occur, resulting in the occurrence of short-circuiting with the metal foil. Further, when using laminate film that has undergone deep drawing to form an area in which the battery element is to be accommodated, the portion of the laminate film that contacts the battery element is usually the portion of the laminate film that has undergone deep drawing. As a result, in spite of the arrangement of heat-seal resin film for preventing short-circuits, the thickness of the layer of heat-seal resin has already been reduced by the deep-drawing process and the arrangement of resin therefore does not obtain the anticipated effect.
Although JP-A-2001-6633 discloses an improvement of the heat resistance of the heat-seal resin of the laminate film, this technique is directed towards improving the reliability of sealing in heat-sealed areas following heat sealing and not to the prevention of short-circuits between the battery element and the metal foil that occur during sealing.